In the past, there has been known a measurement method which is designed to determine a tool diameter of a rotary tool, using a contact type gauge which outputs an electrical signal corresponding to an amount of displacement of a contactor (for example, see Patent Literature 1). In the method described in this Patent Literature 1, a contact type gauge is arranged near the rotary tool, the contactor is made to contact a circumferential surface of the rotary tool while making the rotary tool rotate, and the output value of the contact type gauge at this time is used as the basis to determine the tool diameter.
However, in the method described in the above Patent Literature 1, a contact type gauge which can detect an amount of displacement of the contactor is used, so an expensive sensor is necessary.